In the last video, we
learned that 380,000 years after the Big Bang,
which is still roughly 13.7 billion years
ago, every point-- I shouldn't say every point--
every atom in space that was kind of at this
roughly 3,000 Kelvin temperature was emitting this
electromagnetic radiation. Since every point in space was,
there were points in space, or there was points
in the universe, that that radiation is
only just now reaching us. It has been traveling
for 13.7 billion years. So when we look at
radiation that's been traveling for that long,
we can look at any direction and we'll see this
uniform radiation. And that radiation has been
red-shifted into the microwave range from the
higher frequencies that it was actually emitted at. Now, a question that
might pop in your brain is, well, what happens if
we wait a billion years? Because if we wait
a billion years, if we have 1,000,380,000
years after the beginning of the universe, this stuff
won't just be atoms anymore. It will have started to
condense into actual stars. The universe at
every point in space will no longer be this uniform. We'll actually start having
condensation into stars. So if we move forward a little
bit, the universe will expand. Maybe I'll just draw a half
of it since it's expanded. It's obviously
expanded much more. But now all of a sudden,
we actually have stars. These are no longer
just uniform atoms spread through the universe. We actually have
condensation into stars. And so if you look at
what is being emitted from the points in space from
which we're only now getting this cosmic background
radiation, if we wait a billion years,
the light that we see from those points
in space will not look like this
uniform radiation. It'll start to look
a little bit more like the more mature
parts of the universe. We'll essentially be
looking at the universe a billion years after the Big
Bang, when stars have formed, other structures have formed. So the question is
in a billion years, will this cosmic microwave
background radiation disappear. And I'm using billion just
to arbitrarily use a number. But will it
eventually disappear? And the answer to
that is yes and no. So to think about it, it is
true that this point in space will mature. It will mature in
a billion years. It will no longer
be this uniform-- I guess this uniform haze
of hot hydrogen atoms. But what you have
to think about is there were further
points in the universe. At that same time, there
were further points that were also emitting
this radiation. And the original photons
from those original points still haven't gotten to us. So from those further
out points-- right now, the observable universe
is-- we can only see electromagnetic
radiation that's been traveling for
13.7 billion years. In another billion
years, the universe will be a billion years older. And then there will
be radiation that has been traveling for
14.7 billion years. And so we will start
to observe that. And we'll start to observe that
radiation from the same time period in the universe. It'll just be from further out. Now, what I want
to make clear is, is that since those points
were even further out, where that radiation was emitted,
the stuff that we'll see in a billion years, it
will be even more red-shifted. So at that point, the cosmic
background radiation we see will have longer wavelengths
than the radio spectrum. It will be redder And I should say redder
because we're already more-- would "redder" have two Ds? I've never written "redder." Well, it would be more red
than the microwave radiation. And, of course,
that's a funny thing because microwave
radiation is already more red than actual
visible red light. It has a longer wavelength. Now, this will keep happening. And it'll keep happening. We'll keep getting radiation
as we go further and further into the future. We'll keep getting
radiation from further out points in space. And it'll get more
and more red-shifted. The actual wave lengths of
that electromagnetic light will be bigger, and
bigger, and bigger. Until we really aren't
able to even see it as electromagnetic
light because it'll be red-shifted to infinity. It'll have an
infinite wavelength. And to make that
point clear, I want to show you that at
some point, there will be kind of a threshold
where we can't even get radiation from further out. And let me draw a
diagram of that. So let's say that
this is the universe. Let's say that this is the
universe 13.7 billion years ago, right when that
radiation, what we now see as cosmic microwave
background radiation, right when it started
to be admitted. And let's say that
this is the point in the universe
where we are now. So this is us. Let's say that this is the point
in the universe where we now observe the background
radiation-- or this is one of the points. We obviously could form
a circle around us. It could be any of
these points over here, where the photons, the
electromagnetic radiation that were emitted from this
point, 380,000 years after the beginning
of the universe, is only just now reaching us. So this is the point in
the universe from which we are observing the cosmic
background radiation. And let me be very clear. That point in the
universe has now matured into things that
look-- into stars and galaxies and planets. And if they were to look
at our point in space, they are also going to see
cosmic background radiation from us. It's not like some type
of permanently old place. It's just the light we're
getting from them right now is old light, light that
that point in space emitted way before
it was able to mature into actual structures. So this is the point
in space from which we are receiving cosmic
background radiation right now. I don't want to write all that. It'll take me forever. Now, let's take
another point in space that's whatever
this distance is. Well, it's actually
estimated to be about-- now, it's estimated to be about
46 billion light years. At that time, when things were
just beginning to be emitted, this was only about 36
million light years. And this is a very
rough estimate. I shouldn't even write it down. Because that's really based on
how fast we assume the universe is expanding and all
of that type of thing. But it was just a lot smaller
than 46 billion light years. Now let's go that same distance
again from this point in space. So let me make it clear. This is 380,000 years ago. Now let's fast forward. Let's fast forward-- sorry,
not 380,000 years ago, 380,000 years
after the Big Bang, which is approximately-- it's
still 13.7 billion years ago. So that's then. Now let's look at now. And now I'll just draw
it a little bit bigger. It's actually going to
be much, much bigger now. Now, if we do it a little
bit bigger-- so when I draw it like this,
this is where we are now. This point in space from which
we are only now receiving that cosmic background
radiation is over here. And then this other point in
space is going to be over here. And we saw in the video on the
actual size of the observable universe, not just what it
appears to be based on how long the light's been travelling,
this is now on the order of 46 or 47 billion light years. And so this distance
is also going to be 46 billion light years. Now, every point in
space, back then, was emitting this radiation. We have this uniform radiation. It was just hydrogen atoms
everywhere, these hot hydrogen atoms. Maybe I should just do it in
the color of the radiation. So this guy's
receiving-- I'm just showing it's coming
from this guy. We're only now, 13.7 billion
years in the future, receiving photons from this guy, only
now are we receiving it. And frankly, this
green guy, only now is going to be
receiving photons. When he looks at the point
in space, or the things that he thinks are points
in space out there, he will see that
uniform radiation. And likewise, this guy
over here will only now be receiving photons
from the point in space for where we are now. He'll see the universe
where we are now as it was 380,000 years
after the Big Bang. And same thing from that
point in space, the photons will only just now reach. Now, let's think about it. It took this guy's photons--
let me make it clear. It took him 13.7 billion
years to reach this point over here, which is now 46
billion light years away from us. And the universe
continues to expand. Depending on if the universe
expands fast enough, there's no way that that
photon that got to this guy, will eventually get to this. The universe is expanding
faster than the light can never even catch up to us. And this light will never,
ever, ever get to us. And so there is some
threshold, some distance, from which we will never get
light during this time period or actually from which
we will never, ever get any electromagnetic radiation. So the simple answer is the
cosmic background radiation from this-- or the cosmic
background radiation from this point, yes,
it will start to mature. It won't be as uniform if we go
fast forward 400 million years or a billion years. But we will get uniform
radiation from further out. But it will be even
more red-shifted. And the further forward
we get into the future, the background radiation
we get will be from further and further
out and it will be more and more and
more red-shifted. Until some point,
where it's going to be so red-shifted that
we won't even observe it as electromagnetic radiation. And there's some threshold where
we won't even observe anything anymore because beyond
that, the light wasn't able to actually get to us.